GB2049472A - A method for preventing polymer scale deposition in the polymerization of an ethylenically unsaturated monomer in an aqueous medium - Google Patents

Abstract

The reactor walls are coated, prior to polymerization, with an aqueous coating solution containing a water- soluble anionic dye, a water-soluble cationic dye and a secondary or tertiary alkylamine, of which each of the alkyl groups has from 1 to 6 carbon atoms, followed by drying. Due to the improved wetting ability of the coating solution even on metal surfaces, polymerisation of styrene and other monomers can be performed in a stainless steel polymerization reactor, said polymerization being hitherto conducted in a glass-lined reactor due to the lack of an efficient method for preventing polymer scale deposition on the stainless steel surfaces.

Description

SPECIFICATION A method for preventing polymer scale deposition in the polymerization of an ethylenically unsaturated monomer in an aqueous medium The present invention relates to a method for preventing polymer scale desposition on reactor walls in the polymerization of an ethylenically unsaturated monomer in an aqueous medium.

Ethylenically unsaturated monomers are polymerized in several different ways including suspension or emulsion polymerization in an aqueous medium, solution polymerization, gas-phase polymerization and bulk polymerization according to the kind of monomer and the desired properties ofthe polymer. Among the above named methods of polymerization, several monomers are almost exclusively polymerized in an aqueous medium. Examples of such monomers are vinyl chloride, vinylidene chloride and styrene.

Not only the suspension and emulsion polymerizations but also other polymerization processes suffer from the problem of polymer scale deposition on the walls of the polymerization reactor and other surfaces of the reactor coming into contact with the monomer; this is one of the most difficult problems in the industrial production of polymer products.

The polymer scale deposition on the reactor walls must be prevented as completely as possible because not only of the decrease in the yield of the polymer product and lowering of the cooling capacity of the reactor but also of the degraded quality of the polymer product due to the intermingled fragments of the polymer scale in the product coming off the reactor walls. Further, removing polymer scale requires much time and labor with consequently decreased productivity and increased production costs of the polymer products. What is worse, unreacted monomers are included in a large amount in the polymer scale and the workers undertaking the removal of the scale are exposed to the vapor of the monomer which is very dangerous due to its toxicity to human health.

Various attempts have been proposed hitherto for reducing the amount of polymer scale deposition on the reactor walls. For example, polymer scale deposition on the reactor walls in the aqueous suspension polymerization of vinyl chloride can be reduced by coating the reactor walls, prior to polymerization, with a polar organic compound such as amine compounds, quinone compounds, aldehyde compounds and the like or an organic dye (see, for example, U.S.P. No. 3,669,946). This method is considerably effective in preventing polymer scale deposition but the problem in the method is the use of an organic solvent to dissolve the organic compound or the dye which is not or sparingly soluble in water in order to obtain a coating solution. Needless to say, the use of an organic solvent is undesirable due to toxicity to the human body as well as the danger of fire or explosion.When the organic solvent is replaced with water instead of the organic solvent, if it is ever possible, the expected effect of polymer scale prevention is largely reduced to an impractically low level.

Further, the effectiveness of the above described method is durable when the polymerization of vinyl chloride is carried out as suspension polymerization but, when the polymerization medium contains an emulsifying agent such as in emulsion polymerization or in the combined use of an emulsifying agent with a suspending agent, effectivensss of the coating method is limited and not durable.

Furthermore, there has been proposed no promising method for preventing polymer scale deposition in the polymerization of styrene or copolymerization of styrene and butadiene or styrene, acrylonitrile and butadiene, for example, in an aqueous medium. Therefore, these polymerization procedures are conducted in a glass-lined polymerization reactor in contrast to stainless steel reactors widely used forthe polymerization of vinyl chloride in an aqueous medium.Among the problems of a glass-lined polymerization reactor are that the heat transfer coefficient of the glass-lined reactor walls is much lower than the walls of stainless steel reactors bringing about difficulties in effectively controlling the temperature, that glass-lined walls are susceptible to mechanical damage or fracture resulting in shorter life of the vessel and that a large-size reactor is only obtained with difficulty due to difficulty in fabrication even though the glass-lined surface is less susceptible to polymer scale deposition than stainless steel surfaces.

One of the reasons for the relative ineffectiveness of aqueous coating solutions on stainless steel surfaces is that, due to the remarkably large surface tension of the aqueous solution, the coating solution cannot spread uniformly over the surface, metal surfaces being more or less repellent to aqueous solutions.

Addition of a surface active agent to the aqueous coating solution is effective in improving the spreading of the solution on a metal surface but surface active agents generally increase the polymer scale deposition contrary to the object of coating.

Accordingly, it has been eagerly desired to develop a novel method for the prevention of polymer scale deposition on the reactor walls which is effective not only in the suspension polymerization of vinyl chloride but also in the polymerization of different types of vinyl chloride as well as in the polymerization or copolymerization of a monomer or monomers other than vinyl chloride in an aqueous medium and in which no organic solvent is used for the preparation of the coating solution to be applied on to the reactor walls.

The problem from which the present invention arose was thus to provide an improved method for preventing polymer scale deposition on the reactor walls not only in the aqueous suspension polymerization of vinyl chloride but also in the emulsion polymerization of vinyl chloride as well as polymerization and copolymerization of monomers other than vinyl chloride in an aqueous medium, and particularly such a method which provides a coating layer on the reactor walls prior to polymerization, in which the coating layer is formed by use of an aqueous coating solution so as to be free from the problems in the prior art method using organic coating solutions.

Thus, the method of the invention which is directed to the prevention of polymer scale deposition on the reactor surfaces coming into contact with the monomer or monomers in the polymerization of an ethylenically unsaturated monomer or monomers in an aqueous polymerization medium comprises coating the reactor surface with an aqueous coating solution containing (a) a water-soluble anionic dye, (b) a water-soluble cationic dye and (c) a secondary or tertiary alkylamine compound of which each of the alkyl groups has from 1 to 6 carbon atoms or, preferably, from 1 to 4 carbon atoms and then drying the thus coated reactor surfaces.

Preferred embodiments of the invention will now be described.

In the above described method of the present invention, the component (a) contained in the aqueous coating solution is a water-soluble anionic dye of a class such as water-soluble azo dyes, water-soluble anthraquinone dyes, water-soluble triary!methane dyes, water-soluble xanthene dyes, water-soluble azine dyes, water-soluble quinoline dyes, water-soluble nitro dyes, water-soluble phthalocyanine dyes and the like.

Particular examples of the water-soluble anionic dyes belonging to each of the above mentioned classes are as follows.

Water-soluble azo dyes: C.l. Acid Orange 7; C.l. Acid Red 37; C.l. Acid Red 264; C.l. Acid Blue 113; C.l. Acid Black 1; C.l. Acid Yellow 42; C.l. Acid Blue 158; C.l. Acid Green 12; C.l. Acid Orange 97; C.l. Acid Black 124; C.l. Direct Yellow 50; C.l. Direct Red 37; C.l. Direct Red 2; C.l. Direct Violet 12; C.l. Direct Blue 1; C.I. Direct Brown 1; C.l. Direct Black 77; C.l. Direct Green 1; C.l. Direct Orange 26; C.l. Direct Red 79; C.l. Direct Red 31; 0.1.Direct Black 32; C.i. Direct Yellow 12; C.I. Direct Orange 41; 0.1. Direct Red 113; C.l. Direct Yellow 28; C.l.

Direct Green 26; C.l. Direct Red 81; C.l. Direct Violet 51; C.l. Direct Blue 71; C.l. Direct Brown 37; and C.l.

Direct Black 19.

Water-soluble anthraquinone dyes; C.i. Acid Blue 40; C.l. Acid Red 80; and C.l. Acid Green 41.

Water-soluble triarylmethane dyes: C.l. Acid Blue 1: C.l. Acid Violet 17; and C.l. Acid Green 16.

Water-soluble xanthene dyes: C.l. Acid Red 87; and C.l. Acid Red 52.

Water-soluble azine dyes: C.l. Acid Blue 59; and C.l. Acid Black 2.

Water-soluble quinoline dyes: C.l. Acid Yellow 3; and C.l. Acid Yellow 7.

Water-soluble nitro dyes: C.l. Acid Orange 3; and C.l. Acid Yellow 1.

Water-soluble phthalocyanine dyes: C.l. Direct Blue 86.

The component (b) contained in the aqueous coating solution used in the method in combination with the above defined component (a) is a cationic dye of a class such as water-soluble azine dyes, e.g. C.l. Basic Red 2, C.l. Basic Blue 16 and C.l. Basic Black 2; water-soluble acrydine dyes, e.g. C.l. Basic Orange 14, and C.l.

Basic Orange 15; water-soluble triphenylmethane dyes, e.g. c.l. Basic Blue 1, C.l. Basic Violet 3, C.l. Basic Blue 26, C.l. Basic Violet 14, C.l. Basic Blue 5 and C.l. Basic Blue 7; water-soluble thiazine dyes, e.g. C.l. Basic Blue 9, C.l. Basic Yellow 1, C.i. Basic Blue 24, C.l. Basic Blue 25 and C.l. Basic Green 5; water-soluble methine dyes, e.g. C.l. Basic Red 12 and C.l. Basic Yellow 11; water-soluble diphenylmethane dyes, e.g. C.l. Basic Yellow 2; water-soluble xanthene dyes, e.g. C.l. Basic Violet 10 and C.l. Basic Red 1; water-soluble azo dyes, e.g. C.l. Basic Orange 2 and C.l. Basic Brown 1; and water-soluble oxazine dyes e.g. C.l. Basic Blue 12 and C.l.

Basic Blue 6.

The component (c) contained in the aqueous coating solution used in the method is a secondary or tertiary alkylamine of which each of the alkyl groups has from 1 to 6 carbon atoms or, preferably, from 1 to 4 carbon atoms, as exemplified by diethylamine, dipropylamine, dibutylamine, methylethylamine, methyl isopropylamine, methylbutylamine, ethylpropylamine, ethyl isopropylamine, ethyl isobutylamine, dimethylethylamine, methylbutylamine, ethylpropylamine, ethyl isopropylamine, ethyl isobutylamine, dimentylethylamine, dimethylpropylamine, dimethyl isopropylamine, methyl diethylamine, methyl-ethylpropylamine, The above described components (a), (b) and (c) have, desirably, good or moderate solubility in water but they are not required to be highly water-soluble.A compound having a solubility in water of 0.1% by weight or more can be suitable for the preparation of the aqueous coating solution.

The weight ratio of the individual components (a) to (c) in the aqueous coating solution is of some importance in order to obtain the greatest scale prevention. For example the weight ratio of the component (a) to the component (b) is preferably in the range from 100:5 to 100:100 or, more preferably, from 100:15 to 100:50, whereas the component (c) should desirably be added to the aqueous coating solution in an amount of from 0.01 to 50 parts by weight or, preferably, from 0.05 to 30 parts by weight per 1 part by weight of the total amount of the components (a) and (b).

When the amount of the component (b) relative to the amount of the component (a) is in excess over the above defined range, solid precipitates may sometimes be formed so that no satisfactory coating solution is obtained.

The concentrations of the components (a) to (c) in the aqueous coating solution is not particularly important. It should be noted, however, that the total concentration of the components (a) and (b) is desirably at least 0.01% by weight since, as a matter of course, satisfactory results are not obtained by coating the reactor surfaces with a coating solution too low in concentrations of the components (a) and (b).

On the other hand, no upper limit is given for the concentrations of these dyes in the coating solution and a solution of high concentration can be used if the disadvantages accompanying the use of a coating solution of high concentration is disregarded namely that no particular additional advantages are obtained leading to - lowered economy and that, instead, some inconvenience is caused in the application of the coating solution on to the reactor surfaces. Therefore, it is recommended that the total concentration of the components (a) and (b) in the coating solution does not exceed 5% byweight.

The concentration of the amine compound as the component (c) in the coating solution is preferably in the range from 0.5 to 20% by weight. This is because a lower concentration of the amine than 0.5% by weight cannot give the desired effect of improving the spreading of the aqueous coating solution on metal surfaces whereas excessively large amounts of the amine compound over 20% by weight exhibit no particular additional effects with consequently lowered economy.

The aqueous coating solution used in the method of the invention is readily prepared by merely dissolving the components (a) to (c) in water particularly in the concentrations and in the weight ratios as described above. The first step of the method is coating of the reactor surfaces coming into contact with the monomer or monomers during a polymerization run with the above prepared aqueous coating solution prior to introduction of the monomer or mOnOmerSwater and other ingredients pertaining in the polymerization into the reactor. The surfaces should have been cleaned in advance as far as possible by a conventional method in order to ensure uniform coating with the coating solution. The means for coating is not important and includes spraying, brushing and other conventional methods.The amount of coating is preferably at least 0.001 g/m2 (as dried) to fully exhibit polymer scale prevention.

The reactor surfaces coated with the aqueous coating solution are then dried. the wet surfaces are conveniently and rapidly dried by blowing with hot air at 40 to 10000. Alternatively, the reactor surfaces are heated in advance to 40 to 1000C by a suitable means and the aqueous coating solution is applied on to the heated surface so that coating is directly followed by drying.

After completion of drying, the coated surface is preferably rinsed with water to remove any dissolvable matter leaving the coating films insolubilized by drying.

If convenient and permissible, the reactor surface to be coated with the aqueous coating solution in accordance with the present invention may be undercoated in advance with a conventional coating solution used for the purpose of polymer scale prevention. Such an undercoating treatment is sometimes advisable when further improvement is desired in the reliability and durability of the coating films formed with the aqueous coating solution according to the invention so that they are effective in numbers of repeated polymerization runs.The procedure of the polymerization run per se is not particularly different from conventional runs and the reactor having the surfaces of the wall and other parts coming into contact with the monomer or monomers coated and dried as described above is charged with water as the aqueous polymerization medium, monomer or monomers, polymerization initiators and other ingredients to start the polymerization run.

For example, addition of an alkaline substance into the polymerization mixture is effective for reducing the polymer scale deposition particularly for the polymerization of vinyl chloride in an aqueous medium as is well known in the art. Such an alkaline substance is exemplified by water-soluble compounds of alkali metals or alkaline earth metals including hydroxides, carbonates, hydrogencarbonates, silicates and acetates although the amount of such an alkaline substance should be limited so as not to adversely affect the properties of the polymer products.

As is mentioned above, the method of the present invention is effective for various types of polymerization including not only suspension polymerization of vinyl chloride but also emulsion polymerication of vinyl chloride and polymerization of other ethylenically unsaturated monomers in an aqueous medium. For example, the effectiveness of the method is not affected by the presence of an emulsifying agent in the aqueous polymerization mixture such as sodium laurylsulfate, sodium dodecylbenzenesulfonate, sodium dioctylsulfosuccinate and the like belonging to the class of anionic surface active agents and sorbitan monolaurate, polyoxyethylene alkyl ethers and the like belonging to the class of non-ionic surface active agents.Furthermore, the effectiveness of the method is relatively insusceptible to the influence of the other ingredients such as suspending agents and polymerization initiators as well as several optional additive ingredients such as fillers, stabilizers, lubricants, chain transfer agents, plasticizers and the like.

A diversity of ethylenically unsaturated monomers can be polymerized in an aqueous medium with no or little deposition of polymer scale on the reactor surfaces treated in accordance with the method. The monomers applicable include vinyl halides such as vinyl chloride, vinyl esters such as vinyl acetates and vinyl propionate, acrylic acid and methacrylic acid and esters and salts thereof, maleic acid and fumaric acid and esters thereof, maleic anhydride, dienic monomers such as butadiene, chloroprene and isoprene, aromatic vinyl compounds such as styrene, unsaturated nitriles such as acrylonitrile, vinylidene halides such as vinylidene chloride and vinyl ethers such as vinyl ethyl ether. The method of the present invention is particularly effective for the suspension polymerization or emulsion polymerization of vinyl halides such as vinyl chloride and/or vinylidene halides such as vinylidene chloride or copolymerization of a monomer mixture mainly composed of vinyl halides and/or vinylidene halides.

In addition, the method of the present invention is applicable to the polymerization of styrene, methyl methacrylate and acrylonitrile in an aqueous medium as well as to the emulsion polymerization for the preparation of latices of synthetic rubbers such as SBR, NBR, CR, IR and IIR or ABS resins in a stainless steel polymerization reactor, which polymerizations were performed hitherto in glass-lined polymerization reactors due to the lack of an effective means for preventing scale deposition on the inner wall surfaces of a stainless steel polymerization reactor.

Following are Examples to illustrate the method of the present invention in further detail.

Example 1 (Experiments No. 1 to No.12) Aqueous coating solutions were prepared each by dissolving the components (a), (b? and (c) as indicated in Table 1 below. The weight ratio of the component (a) to component (b) is given in the table. The total concentration of the components (a) and (b) was about 0.1% by weight in each of the solutions and the amount of component (c) given in the table in parts by weight is the amount of the component added to 100 parts by weight of the aqueous solution containing components (a) and (b).

The thus prepared aqueous coating solution was applied by spray coating on to the surfaces of the inner walls of a polymerization reactor of 100 liter capacity and the stirrer thereof coming into contact with the monomer during the polymerization, in a coating amount of9.01-0.1 g/m2 (as dried) followed by drying with heating at 50"C for 15 minutes and washing with water.

Into the thus-treated polymerization reactor were introduced 26 kg of vinyl chloride monomer, 52 kg of deionized water, 26 g of a partially saponified polyvinyl alcohol and 8 g of a,a'-dimethylvaleronitrile and polymerization was conducted at 57 C for 10 hours with agitation.

After completion of the polymerization reaction and discharge of the polymerization mixture out of the reactor, the amount of the polymer scale deposited on the reactor walls was examined to give the results set out in Table 1. As is clear from the results shown in the table, the amounts of the polymer scale deposition in Experiments No. 5 to No. 12 in accordance with the invention were remarkably small in comparison with Experiments No. 1 to No. 4 where no coating was provided (No.1), either one or two of the components (a), (b) and (c) were omitted (No. 2 and No. 3) or ethylamine which is a primary amine was used instead of the secondary or tertiary amine as the component (c) (No.4).In Experiments No. 2 to No. 4 where a coating treatment of the reactor walls was undertaken but the coating solution was not in accordance with the present invention, the scale deposition on the reactor walls was not uniform over the surface and the amounts of scale deposition given in Table 1 are average values over the surface.

TABLE 1 Exp. Water-soluble Water-soluble (a)/(b) Amine compound (c), Polymer No. anionic dye cationic dye weight parts by weight scale (a) (b) ratio deposition g/m 1. None None - None 1200 2. C.I. Acid Black2 None 100/0 None 880 3. None C.I. Basic Orange 14 0/100 None 1000 4. C.I. Acid C.I. Basic 100/25 Ethylamine, 1.0 30 Black 2 Brown 1 5. C.I. Acid C.I. Basic 100/25 Diethylamine, 1.0 3 Black 2 Brown 1 6. C.I. Acid C.I. Basic 100/30 Methyldiethylamine, 1.5 0 Black 124 Blue 16 7. C.I. Direct C.I. Basic 100/33 Methylethyl isobutylamine 2.0 2 Black 32 Orange 15 8. C.I. Basic C.I. Basic 100/50 Ethylpropylamine, 2.5 0 Black 77 Orange 14 9. C.I. Acid C.I. Basic 100/23 Dimethylethylamine, 3.0 1 Orange 97 Green 5 10. C.I. Acid C.I. Basic 100/27 Diethylamine, 1.5 0 Blue 113 Blue 25 11. C.I. Direct C.I. Basic 100/30 Methylethylpropylamine, 1.5 0 Brown 37 Violet 14 12.C.I. Direct C.I. Basic 100/25 Ethylisopropylamine 1.5 1 Blue 71 Red 1 Example 2 (Experiments No. 13 to No. 19) Aqueous coating solutions were prepared in the same manner as in Example 1 with the components (a), (b) and (c) indicated in Table 2 below. The total concentration of the components (a) and (b) was about 0.1% by weight also in this case and the amount of the component (c) shown in Table 2 in parts by weight was the amount of the component per 100 parts by weight of the aqueous solution containing the components (a) and (b).

The thus-prepared aqueous coating solution was applied by spray coating on to the surfaces of the inner walls of a stainless steel polymerization reactor of 120 liter capacity and the stirrer thereof coming into contact with the monomer, in a coating amount of about 0.01-0.1 g/m2 (as dried) followed by drying with heating at 90 C for 10 minutes and thorough washing with water.

Into the thus-treated polymerization reactor were introduced 50 kg of styrene monomer, 43.2 kg of deionized water, 120 g of hydroxyapatite, 0.62 g of sodium hydrogensulfite, 125 g of benzoyl peroxide and 25 g of tert-butyl perbenzoate and the polymerization was conducted at 90 C for 7 hours with agitation.

After completion of the polymerization reaction and discharge of the polymerization mixture out of the reactor, the amount of the polymer scale deposition on the reactor walls was examined to give the results set out in Table 2.

Exp. Water-soluble Water-soluble (a)/(b) Amine compound (c), Polymer No. anionic dye (a) cationic dye weight parts by weight scale (b) ratio deposition, g/m 13 None None - None 280 14 C.I. Acid C.I. Basic Orange 100/30 Diethylamine, 2.0 2 Black 2 Orange 14 15 C.I. Direct C.I. Basic 100/34 Methylethylpropylamine, 2.0 1 Blue 86 Blue 9 16 C.I. Acid C.I. Basic 100/26 Methyl diethylamine, 2.0, 6 Orange 3 Red 1 17 C.I. Direct C.I. Basic 100/23 Diethylamine, 2.0 10 Black 19 Green 5 18 C.I. Acid C.I. Basic 100/40 Ethylpropylamine, 2.0 1 Red 80 Blue 25 19 C.I. Acid C.I. Basic 100/20 Ethyl isopropylamine, 2.0 3 Yellow 7 Violet 10

Claims (6)

1. A method for preventing polymer scale deposition on the walls of a polymerization reactor in the polymerization of an ethylenically unsaturated monomer in an aqueous medium which comprises, prior to the introduction of the monomer, water and other ingredients pertaining in the polymerization into the polymerization reactor, coating the surface of the walls of the polymerization reactor with an aqeuous coating solution containing (a) a water-soluble anionic dye, (b) a water-soluble cationic dye and (c) a secondary or tertiary alkylamine compound of which each of the alkyl groups has from 1 to 6 carbon atoms and drying the thus coated surface.

2. The method as claimed in claim 1 wherein the total concentration of the water-soluble anionic dye and the water-soluble cationic dye in the aqueous coating solution is at least 0.01% by weight, the weight ratio of the water-soluble anionic dye to the water-soluble cationic dye being in the range from 100:5 to 100:100, and the amount of the amine compound is in the range from 0.01 to 50 parts by weight per 1 part by weight of the total amount of the water-soluble anionic dye and the water-soluble cationic dye, the concentration of the amine compound in the aqueous coating solution being in the range from 0.5 to 20% by weight.

3. The method as claimed in claim 2 wherein the weight ratio of the water-soluble anionic dye to the water-soluble cationic dye is in the range from 100:15 to 100:50.

4. The method as claimed in any preceding claim 1 wherein the coating amount of the aqueous coating solution on the surface is at least 0.001 g/m2 as dried.

5. A method as claimed in Claim 1, substantially as described in any of Experiments 5-12 and 14-19.

6. A process for polymerising an ethylenically unsaturated monomer in an aqueous medium wherein the polymerisation is carried out in a reactor which has been treated by method according to any preceding claim.